Amplifiers are used to increase the power of an electrical signal. One common type of amplifier is an operational-amplifier (op-amp), a high-gain voltage amplifier having a pair of differential inputs and an output. The op-amp amplifies the difference between the voltages at the inputs, typically by a factor of hundreds or thousands, and presents the amplified difference at the output. Op-amps may be constructed using a number of circuit configurations in order to configure the op-amp for use in performing various functions. For instance, audio devices commonly utilize audio op-amps that are configured particularly for the amplification of audio signals within the range of human hearing.
A single op-amp is frequently unable to provide the full amount of amplification that is required by the system utilizing the output of the op-amp. Even if a single op-amp is able to provide the required amplification, the op-amp may be unable to do so within the operational parameters required by the host system. Consequently, op-amps commonly utilize multiple, cascaded stages of amplification, with each stage comprising of one or more amplification circuits. The output of each stage is coupled in some way to the input of the next stage. Certain features of the amplifier may be implemented as additional, non-amplifying stages. An op-amp typically consists of at least two stages: an input stage and an output stage. In a differential amplifier, the input stage has the task of deriving the difference between the two inputs. The input stage or other intervening stages provide the gain used to amplify the difference between the two input signals. The output stage modifies the amplified signal such that the output satisfies the requirements set forth by the host system.
Audio op-amps are used in a wide variety of audio devices. The fidelity and other output characteristics that are required in the output signal generated by an audio op-amp vary significantly based on the audio device application. For instance, audio op-amps used in home theater equipment will have different current, voltage and impedance requirements than an audio op-amp used to provide audio for a set of headphones. In all applications, audio op-amps seek to provide an output signal that replicates the input signal with as little distortion as possible. A common measure of the output fidelity of an audio op-amp is the total harmonic distortion (THD) in the output. The lower the THD the more accurately the output of the op-amp replicates the input to the audio op-amp.
The output stage of an op-amp may utilize a “push-pull” arrangement of transistors or other switching elements to provide current to components of the output stage. In some push-pull configurations, pairs of complementary transistors alternatively source or sink the current of the audio signal. In other push-pull configurations, the pairs of transistors alternate between high and low voltages for powering the components of the op-amp. The use of push-pull arrangements allows for efficient use of power and promotes cancelling of noise and non-linear distortions.
Crossover distortion is a particular variant of THD that can result from the use of push-pull configurations of switching elements in an amplifier. In a push-pull arrangement, the switching elements are used in pairs, with the switching elements that comprise a pair each providing amplification for only a half-cycle of the input signal. Crossover distortions arise in the output of a push-pull circuit due to non-synchronized transitions from one switching element to another at each half-cycle boundary. For instance, lack of synchronization during these transitions may result in intervals where both switching elements of a pair are operating simultaneously or intervals where neither switching element in a pair is operating. These non-synchronized transitions between push-pull elements of an amplifier create non-linear regions (i.e., crossover distortions) in the output of the amplifier.
During operation of an op-amp, an output stage consumes current from a power supply. A portion of this current, known as the quiescent current, is used to bias the internal components of the output stage. The quiescent current is the current required to bias all transistors on, while the input is shorted to ground. The quiescent current provides sufficient current for the transistors to maintain the amplifier components in an on state such that they can quickly resume operations. Low quiescent current is desirable because it reduces power consumption when the amplifier is operating at a light load, or with no load.
An audio op-amp output stage is configured to generate an audio signal output that can drive a speaker system of a specified impedance. In light of the low voltage utilized in certain speaker systems, such as headphones, crossover distortion in the audio signal can be particularly problematic. Increasing quiescent current is one way to reduce crossover distortion, but increasing quiescent current may be problematic for battery powered applications. Consequently, there is a need for an audio op-amp that operates using a low quiescent current and is capable of driving low impedance speaker systems while minimizing crossover distortion of the audio signal.